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Metabolic Responses to Exercise Intensity: Fuel Selection and Physiological Factors Explained

This chapter explores the metabolic responses to exercise intensity, discussing oxygen deficit, lactate threshold, fuel selection regulation, and ATP production pathways. It delves into the relationship between exercise intensity and bioenergetic pathways, emphasizing the importance of carbohydrate metabolism in fat metabolism. The chapter also covers oxygen debt, excess post-exercise oxygen consumption, and the shifts in fuel utilization during different exercise intensities. Mechanisms influencing oxygen uptake, lactate threshold, and factors affecting VO2max are detailed, highlighting the physiological adaptations during various exercise durations and intensities.

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Metabolic Responses to Exercise Intensity: Fuel Selection and Physiological Factors Explained

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  1. Chapter 4Exercise Metabolism EXERCISE PHYSIOLOGY Theory and Application to Fitness and Performance, 6th edition Scott K. Powers & Edward T. Howley

  2. Objectives • Discuss the relationship between exercise intensity/duration and the bioenergetic pathways • Define the term oxygen deficit • Define the term lactate threshold • Discuss several possible mechanisms for the sudden rise in blood-lactate during incremental exercise • List the factors that regulate fuel selection during different types of exercise

  3. Objectives • Explain why fat metabolism is dependent on carbohydrate metabolism • Define the term oxygen debt • Give the physiological explanation for the observation that the O2 dept is greater following intense exercise when compared to the O2 debt following light exercise

  4. Rest-to-Exercise Transitions • Oxygen uptake increases rapidly • Reaches steady state within 1-4 minutes • Oxygen deficit • Lag in oxygen uptake at the beginning of exercise • Suggests anaerobic pathways contribute to total ATP production • After steady state is reached, ATP requirement is met through aerobic ATP production

  5. The Oxygen Deficit Fig 4.1

  6. Differences in VO2 Between Trained & Untrained Subjects Fig 4.2

  7. Recovery From Exercise Metabolic Responses • Oxygen debt or • Excess post-exercise oxygen consumption (EPOC) • Elevated VO2 for several minutes immediately following exercise • “Fast” portion of O2 debt • Resynthesis of stored PC • Replacing muscle and blood O2 stores • “Slow” portion of O2 debt • Elevated heart rate and breathing,  energy need • Elevated body temperature,  metabolic rate • Elevated epinephrine & norepinephrine,  metabolic rate • Conversion of lactic acid to glucose (gluconeogenesis)

  8. Oxygen Deficit and Debt During Light-Moderate and Heavy Exercise Fig 4.3

  9. Removal of Lactic Acid Following Exercise Fig 4.4

  10. Fig 4.5

  11. Metabolic Response to Exercise Short-Term Intense Exercise • High-intensity, short-term exercise (2-20 seconds) • ATP production through ATP-PC system • Intense exercise longer than 20 seconds • ATP production via anaerobic glycolysis • High-intensity exercise longer than 45 seconds • ATP production through ATP-PC, glycolysis, and aerobic systems

  12. Metabolic Response to Exercise Prolonged Exercise • Exercise longer than 10 minutes • ATP production primarily from aerobic metabolism • Steady state oxygen uptake can generally be maintained • Prolonged exercise in a hot/humid environment or at high intensity • Steady state not achieved • Upward drift in oxygen uptake over time

  13. Upward Drift in Oxygen Uptake During Prolonged Exercise Fig 4.6

  14. Metabolic Response to Exercise Incremental Exercise VO2 – Ability to Deliver and Use Oxygen • Oxygen uptake increases linearly until VO2max is reached • No further increase in VO2 with increasing work rate • Physiological factors influencing VO2max • Ability of cardiorespiratory system to deliver oxygen to muscles • Ability of muscles to use oxygen and produce ATP aerobically

  15. Changes in Oxygen Uptake With Incremental Exercise Fig 4.7

  16. Lactate Threshold • The point at which blood lactic acid suddenly rises during incremental exercise • Also called the anaerobic threshold • Mechanisms for lactate threshold • Low muscle oxygen • Accelerated glycolysis • Recruitment of fast-twitch muscle fibers • Reduced rate of lactate removal from the blood • Practical uses in prediction of performance and as a marker of exercise intensity

  17. Identification of the Lactate Threshold Fig 4.8

  18. Mechanisms to Explain the Lactate Threshold Fig 4.10

  19. Other Mechanisms for the Lactate Threshold • Failure of the mitochondrial hydrogen shuttle to keep pace with glycolysis • Excess NADH in sarcoplasm favors conversion of pyruvic acid to lactic acid • Type of LDH • Enzyme that converts pyruvic acid to lactic acid • LDH in fast-twitch fibers favors formation of lactic acid

  20. Effect of Hydrogen Shuttle and LDH on Lactate Threshold Fig 4.9

  21. Estimation of Fuel Utilization During Exercise • Respiratory exchange ratio (RER or R) • VCO2 / VO2 Fat (palmitic acid) = C16H32O2 C16H32O2 + 23O2 16CO2 + 16H2O + ?ATP R = VCO2/VO2 = 16 CO2 /23O2 = 0.70 Glucose = C6H12O6 C6H12O6+ 6O2 6CO2 + 6H2O + ?ATP R = VCO2/VO2 = 6 CO2 /6O2 = 1.00

  22. Estimation of Fuel Utilization During Exercise • Indicates fuel utilization • 0.70 = 100% fat • 0.85 = 50% fat, 50% CHO • 1.00 = 100% CHO • During steady-state exercise • VCO2 and VO2 reflective of O2 consumption and CO2 production at the cellular level

  23. Exercise Intensity and Fuel Selection • Low-intensity exercise (<30% VO2max) • Fats are primary fuel • High-intensity exercise (>70% VO2max) • CHO are primary fuel • “Crossover” concept • Describes the shift from fat to CHO metabolism as exercise intensity increases • Due to: • Recruitment of fast muscle fibers • Increasing blood levels of epinephrine

  24. Illustration of the “Crossover” Concept Fig 4.11

  25. Exercise Duration and Fuel Selection • During prolonged exercise, there is a shift from CHO metabolism toward fat metabolism • Increased rate of lipolysis • Breakdown of triglycerides into glycerol and free fatty acids (FFA) • Stimulated by rising blood levels of epinephrine

  26. Shift From CHO to Fat Metabolism During Prolonged Exercise Fig 4.13

  27. Interaction of Fat and CHO Metabolism During Exercise • “Fats burn in a carbohydrate flame” • Glycogen is depleted during prolonged high-intensity exercise • Reduced rate of glycolysis and production of pyruvate • Reduced Krebs cycle intermediates • Reduced fat oxidation • Fats are metabolized by Krebs cycle

  28. Sources of Fuel During Exercise • Carbohydrate • Blood glucose • Muscle glycogen • Fat • Plasma FFA (from adipose tissue lipolysis) • Intramuscular triglycerides • Protein • Only a small contribution to total energy production (only ~2%) • May increase to 5-15% late in prolonged exercise • Blood lactate • Gluconeogenesis via the Cori cycle

  29. Effect of Exercise Intensity on Muscle Fuel Source Fig 4.14

  30. Effect of Exercise Duration on Muscle Fuel Source Fig 4.15

  31. The Cori Cycle:Lactate As a Fuel Source Fig 4.16

  32. Chapter 4Exercise Metabolism

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